The present invention concerns a process for the cyclization of orthobenzoylbenzoic acid (OBB acid) in the presence of superactive bleaching earth in order to produce anthraquinone.
It is known to form anthraquinone from OBB acid by heating this acid compound in the presence of concentrated sulfuric acid or an oleum. Such a method is, for instance, reported by Arthur I. VOGEL in "Practical Organic Chemistry", 3rd edition, page 740. The sulfuric acid plays the role of catalyst and solvent for the reaction.
The concentrated sulfuric acid can be replaced by concentrated phosphoric acid or by phosphorus pentoxide. In each of the methods cited above, the large quantity of reagent utilized decreases the efficiency of the corresponding process and creates important pollution problems due to the discarded acid material.
Japanese Patent Application No. 49.7260/74 proposes working with a lesser amount of sulfuric acid, but the operation must then take place in a vacuum and at an elevated temperature above 300.degree. C. This causes operating risks and a high cost for the equipment due to corrosion inherent in the conditions of implementation of such a process. In addition, the recovery by sublimation of the anthraquinone formed can take place only after neutralization of the sulfuric acid with sodium carbonate.
U.S. Pat. No. 2,842,562 also describes a process utilizing a reduced quantity of sulfuric acid. The ring closure (cyclization) of OBB acid takes place at 260.degree. C. in the presence of a third solvent such as trichlorobenzene, with the water formed during the reaction being eliminated by distillation in the form of an azeotropic mixture with the third solvent. The use of the latter solvent complicates the equipment and is uneconomical since the anthraquinone must be recovered by separation from the solvent by steam distillation.
U.S. Pat. No. 2,174,118 proposes hydrofluoric acid as a cyclization catalyst for OBB acid. The use of such a reactant involves major drawbacks due to its very nature; namely, corrosion by the water/hydrofluoric acid combination formed during the course of the operation, difficult recovery of the hydrofluoric acid by dehydration of this same combination, and finally the necessity of operating the ring closure of the OBB acid under pressure.
U.S. Pat. No. 4,304,727 describes a process in which a perfluorinated resin in suspension in an inert organic solvent is used as a cyclization catalyst. Although the recovery of the catalyst is thus facilitated, the OBB acid conversion and the anthraquinone yield are low, for example, equal respectively to 60% and 78%. Additionally, it is necessary to proceed with the separation of the organic solvent from the anthraquinone.
The Japanese Patent Application No. 49.6240/74 recommends cyclizing the OBB acid by heating at 360.degree. C. in the presence of activated clay. This method presents drawbacks rendering it commercially unfeasible due to the necessity of operating in a vacuum, slow ring closure rate, necessity of an intimate mixture of the OBB acid and of the activated clay, and a low practical yield following the absorption by the activated clay of nearly one quarter of the anthraquinone formed. The recovery of the anthraquinone thus absorbed would require costly extraction operations from the clay with an organic solvent and then evaporation of the solvent.
French Utility Certificate No. 76.18956, published under No. 2,314,913, describes the cyclization of OBB acid in the presence of oxygenated compounds of aluminum and silicon, preferably by heating the OBB acid with aluminum silicate in powder form. When the process is conducted in suspension in an organic solvent, the elimination of the solvent and then of the catalyst is complicated and costly whether it is carried out in a fixed bed or in a fluidized bed. In the first case one risks a rapid clogging of the catalyst, while in the second case one risks coming up against a delicate separation of the fine particles of catalyst from the anthraquinone. The best yields of cyclization are also achieved when the latter process is carried out in a vacuum.